If sucrose solutions of such concentrations as may may come in consideration in conjunction with the production of sucrose on an industrial scale, are subjected to spray drying without taking special precautions, the sucrose will be supercooled and the resulting product is an incompletely crystallized substance which adheres to the walls of the spray drier.
However, it is known from British Specification No. 1,191,908 that grains of crystalline sugar can be obtained by spray drying a massecuite obtained by cooling-crystallization of a sucrose solution. The performance of such cooling-crystallization is, however, very time-consuming (from 3 to 6 hrs) and requires, therefore, equipment of considerable dimensions. Moreover, in conjunction with this method there occurs a certain loss of heat since not only the free heat within the solution but also the crystallization heat liberated, cannot be utilized but is carried away with the coolant.
British Specification No. 1,072,816 discloses an agglomeration method where a suspension of particles is spray dried in a liquid containing binding agent, for the purpose of producing a granular product where the individual grains consist of a plurality of particles cemented together by a relatively small amount of binding agent. The particle suspension spray dried is produced either by cooling-crystallization or by adding particles to a liquid. In either case the suspension contains so many particles that up to 90% or above of the substance in question is in particle form, and the primary object is to cement the fine particles together to form larger grains. The examples contained in the Specification do only prove the applicability of the method in conjunction with particles of lactose or fumaric acid and with a binding agent of lactose, glucose or fumaric acid, and on the basis thereof it is not possible to draw any conclusions as to the applicability of the method in conjunction with sucrose, and particularly not because it is known inter alia from the below-mentioned British Specification No. 1,240,691, that sucrose solutions are more sticky and difficult to spray dry than other sugars, such as glucose. To this is added that the person skilled in the art has felt no urge to investigate the possible applicability of the method in conjunction with the recovering of sucrose on an industrial scale since, as mentioned, the method comprises either the addition of an extremely large amount of fine particles to the solution to be dried, or the use of a complex and time-consuming cooling-crystallization for obtaining the required amount of crystals in the suspension.
Furthermore, it is known from British Specification No. 1,282,878 to spray dry sucrose solutions by injecting sucrose particles dispersed in air into the cloud of atomized sucrose solution within the spray drier, and these sucrose particles may consist of recycled material. However, the injection of solid particles dispersed in air into the spray drier involves certain practical drawbacks, and to ensure that practically all atomized liquid droplets, while suspended in air, get into sufficient contact with solid particles, it will be necessary to inject a substantially greater number of solid particles in excess of those actually getting into contact with the droplets, for which reason the number of solid particles must be very large. The injection of this large amount of particles requires considerably volumes of air, for which reason the heat economy of the drying process is highly deteriorated when using this arrangement.
A modification of this method is object of British Specification No. 1,387,062. In this method a very great number of the sucrose particles produced are recycled to the spray drier so that said particles are distributed along the outer periphery of the drying zone. The result hereof, it is alleged, is that a free-flowing dry product can be discharged directly from the spray drier, and that the problems with sucrose deposits on the spray drier walls are diminished. However, this method is also encumbered with the shortcomings referred to in connection with the process disclosed in British Specification No. 1,282,878.
British Specification No. 1,240,691 refers to a method where, in addition to the injection of sucrose crystals into the drying air, sucrose crystals are also admixed to the solution to be spray dried. Also this method is of course encumbered with the shortcomings referred to in connection with British Specification No. 1,282,878.